In the manufacture of telecommunications cable a protective plastic jacket is often extruded over a filamentary cable core. This is done with an extruder of the type having a crosshead. The crosshead serves to redirect and to reconfigure a stream of plastic fluid of solid, usually cylindrical shape into a tubular shape about the cable. This is performed with the use of a diverter tube, also sometimes referred to as a compensation tube, that is seated within a cylindrical cavity of a crosshead block. The interior surface of the diverter tube is sized to receive a cable core passed linearly therethrough, or to receive a core tube holder through which the cable core is passed, while its generally cylindrical exterior surface is provided with raised lands that define channels. These channels are shaped so as to redirect the flow of plastic fluid introduced into the crosshead some 90.degree. and to divide it into two or more streams that are routed to a plurality of diverter tube channel orifices spaced radially along the cylindrical cavity. From here the configuration of the raised lands is such as to permit the plastic fluid to spread into a tubular confluence that is drawn down upon the cable core in the extruder die.
As the fluidic plastic material must ordinarily follow a flow path having changes in both direction and path size, flow imbalance conditions are inherently created. These flow imbalances, wherein various portions of the flow at any one point along the path travel at different speeds, create radial variations in the thickness of the wall of the tubular confluence once it has solidified into a jacket. Since some minimum wall thickness is required for proper cable performance, these circumferential variations in jacket thickness must be compensated for by an increase in the average wall thickness. This, of course, increases manufacturing costs.
The just described problem of wall thickness variations in extruded, tubular jackets has heretofore been recognized and attempts made at providing solutions. These solutions have taken the form of crosshead designs that divide the stream of plastic fluid delivered to the crosshead into several smaller branch streams that are routed radially about the cable and then recombined into a tubular confluent stream to equalize the flow rate of plastic radially onto the cable. This has been done on a volumetric or flow rate basis neglecting pressure and velocity distributions in the plastic stream. Though such designs have improved concentricity and roundness of tubular extrusions, they have not been satisfactory when the plastic utilized has been of a highly viscous or elastic type. Molten polypropylene or medium density polyethylene, for example, when moving through a conduit is subjected to shear stresses that result in substantial velocity and pressure gradients, particularly in channel bends and enlargements, which do not readily return to steady state fluid flow conditions.
Two examples of the just described approaches are shown in U.S. Pat. Nos. 3,579,731 and 3,860,686. The former patent here discloses a crosshead having a compensation or diverter tube formed with an annular restriction located downstream of a fluid delivery port. The restriction has an axial length that tapers from an axially long surface located radially adjacent the delivery port to an axially short surface located radially opposite the delivery port. This construction has been found to perform well where the taper is designed for a specific flow rate of a plastic fluid of known viscosity. Its effectiveness, however, is diminished significantly when plastic fluids of other viscosities are used or where other flow rates are employed. The latter patent provides a lengthening of the flow paths within the crosshead so that the plastic has a longer time in which to dampen out flow pattern distrubances and to achieve steady state flow conditions before contacting an advancing wire. Specifically, a highly viscous fluent plastic stream is divided into smaller streams that are routed along paths that have both radially and axially oriented path segments that ultimately merge into an annular confluence. This methods does provide enhanced flow balance characteristics but achieves such with apparatus that is complex and costly. Furthermore, use has been limited to the application of extruded insulation about wires. It would be quite difficult to use this in the manufacture of cables due to the size limitation which flow and pressure requirements impose on cable jacket extruders.
It thus is seen that the need remains for the development of practical and cost effective methods and means for extruding plastic jackets of uniform tubular wall thicknesses about cables and the like. It is this problem to which the present invention is primarily directed.